1. Field of the Invention
Apparatus and methods for characterization of wafers and other objects, such as in the manufacturing of semiconductor wafers.
2. Description of Related Art
Many micro- and nano-scale products, such as integrated circuit (IC) chips and microelectromechanical systems (MEMS), are made according to well known processes. The starting material for such products is typically a thin substrate wafer, such as a silicon wafer substrate. Key steps in manufacturing ICs and MEMS may include film deposition, photolithographic patterning of features in the film, and processing to remove material and create three-dimensional patterned features on the wafer surface. The film deposition, patterning, and processing steps are repeated with sequences of materials to form the functional features of an IC or MEMS device. Current IC chips now have features of less than 50 nanometers.
The substrate wafers are processed to produce such products in a range of sizes, typically from two inches to twelve inches in diameter. Between the various film deposition, patterning, and processing steps, it is common to perform certain characterization steps for quality control of these steps, and of the overall final product. For example, both before and after a film deposition, the wafer may be imaged and weighed. By comparing the images before and after the film deposition, defects in the film may be identified. The thickness of the film and other physical characteristics of the process steps can be inferred by comparing the weights of the wafer before and after the film deposition.
When imaging a wafer, in order to obtain “before” and “after” images that can be easily compared with each other, the wafer must be aligned relative to a fiduciary mark (such as notch or flat) before image capture by a camera, or other optical device. Wafers in need of characterization are typically delivered to a characterization station in a “cassette,” which is a carrying device having multiple closely-spaced slots in which the wafers are disposed.
Referring to FIG. 1, a characterization station 10 may be comprised of a wafer handling robot 12, a wafer aligner 14, and a wafer imager 16, all of which may be disposed upon a base platform 18. The robot 12, aligner 14, and wafer imager 16 may be in communication with and controlled by a computer 20, which may also acquire image data from the wafer imager 16.
To perform the characterization process of imaging a wafer, the wafer 2 is removed from a cassette 22 by the robot arm 13, and disposed upon a chuck (not shown) of the wafer aligner 14. The aligner 14 aligns the wafer 2 by rotating it past an optical sensor (not shown) that is in communication with the computer 20. The center of the wafer 2 is located by the computer based on data from the optical sensor. The chuck of the aligner 14 is temporarily lowered, placing the wafer 2 onto a set of stationary support posts, the aligner chuck is moved so that the center of rotation of the chuck is aligned with the center of the wafer 2, and then the aligner chuck is raised and re-engaged with the wafer 2. Thus any translational and angular placement error by the robot is corrected prior to imaging the wafer 2. The wafer 2 is then rotated to a desired position relative to its notch or flat under the wafer imager 16. The wafer imager 16 may include a separate wafer ID reader, which reads and recognizes a number or other code that is formed at a specific location on the wafer surface. A full image of the wafer is captured, associated with the wafer number, and stored in a memory of the computer 20. The image may be further processed with software. The wafer 2 is then removed from the aligner and returned to its slot in the cassette 22. The characterization process steps of transporting from the cassette 22 to the aligner 14, aligning, code reading, imaging, and returning from the aligner 14 to the cassette 22 are repeated for the remaining wafers in the cassette 22.
Subsequently, the cassette 22 of wafers 2 may be moved to a second characterization station (not shown), which includes a weighing scale, and another robot. Thus the steps required to obtain a weight of the wafer 2 are transporting the wafer from the cassette 22 to the scale, weighing the wafer 2, and returning the wafer 2 from the scale to the cassette 22. Alternatively, the characterization station 10 may be provided with a scale (not shown) that is disposed on the base 18, in which case, the robot 12 may move the wafer 2 from the aligner 14 to the scale, and then from the scale back to the cassette 22. A second dedicated robot (not shown) may be provided to perform the transfers to and/or from the scale.
Regardless of whether the wafer weighing operation is performed at a separate characterization station, or by a scale provided at the station 10, there are problems resulting from operating in this manner. Semiconductor manufacturing and other nanoscale processes are performed in clean rooms. A particle-free environment is maintained to the greatest extent possible in order to minimize defects on the wafers. Generally, the more separate process operations that are performed, the more particulate contaminants will be generated. Additional wafer handling increases the likelihood of defect causing contamination or physical damage to the wafers, resulting in costly yield loss. Additionally, performing the operations of aligning, imaging, and weighing separately slows manufacturing throughput, and requires additional capital equipment and plant floor space, both of which also increase unit product cost.
In addition to the problems of handling the wafers to and from the weighing station 10, there are problems with determining wafer characteristics based on wafer weight. Accurate determination of process results such as film thickness based on weight are subject to influence from many variables. These variables may be properties of the materials, used or environmental factors, such as temperature and humidity. Present state of the art requires that variables be either determined along with the wafer weight or recorded with the wafer weight data. In addition, some of the material properties may not be known with high enough reliability to ensure accurate determinations based on the wafer weight.
What is needed is a single characterization station which can perform all of the wafer characterization operations of aligning, imaging, and weighing in a simple and rapid manner.
Along with wafer imaging, current wafer process control relies on instruments to measure physical properties of the deposited films and features. These instruments are designed to reliably characterize absolute properties at specific points across the wafer. The results are compared with expected values and a quality determination is made. However these measurements require additional instruments and are more time consuming. Furthermore, each physical measurement is dependent upon a high degree of support due to strict calibration and parameter control requirements. A wafer characterization system, algorithms and methods are needed to provide simple ways to achieve process control results without complex computations or apparatus.